1. Technical Field
The present invention relates to a mobile station and a base station in a mobile communication system with transmission power control, a transmission power control method for mobile stations, and a recording medium containing a program for achieving this transmission power control method.
2. Background Art
In mobile radio systems, a service area is formed by providing a number of radio zones, which are circular areas centered around base stations. For the purposes of covering the service area fully, the base stations are provided so that a plurality of radio zones will overlap at the edges of the radio zones.
Since the radio zones are centered around base stations, the intensity of a desired signal to be received by a mobile station in a service area will be high in the vicinity of the base station but low near the radio zone edges. Since the communication quality will be reduced if an interfering signal is stronger than the desired signal, efforts have been made in existing mobile radio systems to retain the service quality by performing handover to another radio zone capable of achieving better communication quality when a mobile station has moved to the edge of a radio zone.
On the other hand, in CDMA (code division multiple access) systems, transmission power control is performed to always keep the transmission power of mobile stations as low as possible, for the purposes of retaining capacity and quality and reducing interference with other stations. The transmission power control in these mobile stations is achieved by closed-loop control, wherein the transmission power is increased and decreased in accordance with instructions given by the content of a single-bit transmission power control signal (hereinafter referred to as a TPC bit) sent from the base station.
Additionally, in the case of so-called soft handover, the mobile station simultaneously connects to a plurality of connectable base stations, and begins control of the switching between radio zones. At this time, the mobile station detects the TPC bits sent from the plurality of base stations by the above-mentioned closed-loop control, and decides and controls its own transmission power based on the content of the detected TPC bits. Here, an example of the transmission power control during soft handover in a mobile station is shown in FIG. 13. In the example shown in FIG. 13, the transmission power is increased only when the content of the TPC bits from all of the connected base stations gives instructions for a transmission power increase (the value of the TPC bit is xe2x80x9c1xe2x80x9d), and otherwise, the transmission power is decreased.
However, during soft handover, the mobile station is not capable of receiving the signals from all of the connected base stations with the same quality. That is, there is a possibility of the reception quality of the signal from a certain base station being degraded. Degradations in the signal reception quality from a base station mean an increase in the error rate for transfer of the TPC bit from that base station. As described above, since a substance of the transmission power control of a conventional mobile station is decided by combining the contents of all TPC bits detected by the mobile station, an increase in the error rate for transfer of the TPC bit from one base station will directly lead to an increase in the probability that inappropriate transmission power control will be performed.
Moreover, in the above-described example, wherein the transmission power is increased only when the contents of the TPC bits from all of the connected base stations are xe2x80x9c1xe2x80x9d (when the TPC bits from all of the connected base stations give instructions for transmission power increase) as shown in FIG. 13, the probability that the transmission power will be made smaller than necessary will rise when the error rate for transfer of a TPC bit from a single base station increases. This is due to the fact that if an error arises in a TPC bit from a base station B when the combination of TPC bits sent from a base station A and base station B is xe2x80x9c11xe2x80x9d (transmission power increase), the reception pattern will become xe2x80x9c10xe2x80x9d, and will result in a transmission power decrease, whereas if an error arises in the a TPC bit from base station B when the combination of TPC bits sent from base station A and base station B is xe2x80x9c00xe2x80x9d, xe2x80x9c10xe2x80x9d or xe2x80x9c01xe2x80x9d (transmission power decrease), the reception pattern will become xe2x80x9c01xe2x80x9d, xe2x80x9c11xe2x80x9d or xe2x80x9c00xe2x80x9d, and will not always result in a transmission power increase. This effect can be expected to become more pronounced when the number of connected base stations is large.
In the end, as is clear from what is described above, the transmission power control of a conventional mobile station has a problem in that when the reception quality of the signals from any one of the base stations is low, then appropriate power control cannot be performed during soft handover, and the communication quality is reduced.
The present invention has been made in view of the above background, and has the object of offering a mobile station and a base station, a transmission power control method for a mobile station, and a recording medium containing a program for achieving this transmission power control method, which are capable of appropriately controlling the transmission power of the mobile station during soft handover, even if the reception qualities of the signals from the connected base stations are not the same.
In order to resolve the above-mentioned problems, in a first aspect, the present invention comprises a transmission power control method for a mobile station simultaneously connected to a plurality of base stations. The method comprises a receiving step for receiving a transmission power control signal from each of said plurality of base stations in said mobile station; a reliability level acquiring step for acquiring a reliability level for each of said plurality of base stations; an object signal acquiring step for determining an object signal from said reliability levels acquired in said reliability level acquiring step and said plurality of transmission power control signals received in said receiving step; a control content deciding step for deciding a control content of the transmission power of said mobile station based on said object signal determined in said object signal acquiring step; and a control step for controlling the transmission power of said mobile station in accordance with the control content decided in said control content deciding step. According to this transmission power control method, the object signal is determined in the mobile station in consideration of not only the plurality of transmission power control signals, but also of the reliability level of each of the plurality of base stations, and the transmission power control is performed on the basis of this object signal. Therefore, the influence of a transmission power control signal for which an error has occurred during transfer can be made small. As a result, transmission power control can be performed with high precision, and the degradation of the communication quality and the degradation of the subscriber capacity can be prevented.
Additionally, in the above-described transmission power control method, it is possible, in said mobile station, to acquire a parameter based on a downward signal from each of said plurality of base stations, and generate a reliability level with respect to each of said plurality of base stations in accordance with said parameter; or in each of said plurality of base stations, to acquire a parameter based on an upward signal from said mobile station, and send said parameter to said mobile station, and in said mobile station, generate a reliability level with respect to each of said plurality of base stations in accordance with the parameter sent from each of said plurality of base stations; or in each of said plurality of base stations, to acquire a parameter based on an upward signal from said mobile station, generate a reliability level for itself based on said parameter, and transmit the generated reliability level to said mobile station; and in said mobile station, receive the reliability level sent from each of said plurality of base stations. In particular, when the parameters are acquired in each of said plurality of base stations, transmission power control can be performed with higher precision in the mobile station.
Furthermore, the following characteristics can also be appended to the above-described transmission power control method.
A weighting step for weighting the transmission power control signals received in said receiving step according to said plurality of reliability levels acquired in said reliability level acquiring step can be provided; said object signal acquiring step can include generating said object signal by combining the plurality of transmission power control signals weighted in said weighting step.
The object signal acquiring step can include selecting one signal from among the transmission power control signals received in said receiving step based on said plurality of reliability levels acquired in said reliability level acquiring step, and taking the selected signal as said object signal.
The control content deciding step can include comparing said object signal determined in said object signal acquiring step with two preset threshold values, and selecting and determining the control content of said mobile station from among the three stages of an increase, maintenance or decrease of the transmission power based on the results of the comparison.
The control can be performed, in said control content deciding step, by changing the transmission power of said mobile station by a control amount in accordance with said object signal determined in said object signal acquiring step.
Additionally, in order to resolve the above-mentioned problems, in a second aspect, the present invention comprises a mobile station capable of simultaneously connecting with a plurality of base stations. The mobile station comprises receiving means for receiving a transmission power control signal from each of said plurality of base stations; reliability level acquiring means for acquiring a reliability level with respect to each of said plurality of base stations; object signal acquiring means for determining an object signal based on said plurality of transmission power control signals received from said receiving means and said plurality of reliability levels acquired from said reliability acquiring means; control content deciding means for deciding a control content of the transmission power of said mobile station based on said object signal determined by said object signal acquiring means; and control means for controlling the transmission power in accordance with said control content decided by said control content deciding means. According to this mobile station, the object signal is determined in the mobile station in consideration of not only the plurality of transmission power control signals, but also of the reliability level of each of the plurality of base stations, and the transmission power control is performed on the basis of this object signal. Therefore, the influence of a transmission power control signal for which an error has occurred during transfer can be made small. As a result, transmission power control can be performed with high precision, and the degradation of the communication quality and the degradation of the subscriber capacity can be prevented.
Additionally, in the above-described mobile station, the reliability level acquiring means can acquire a parameter based on a signal from each of said plurality of base stations received by said receiving means, and generate a reliability level with respect to each of said plurality of base stations in accordance with said parameter. The reliability level acquiring means can generate a reliability level with respect to each of said plurality of base stations in accordance with a parameter from each of said plurality of base stations received by said receiving means. The reliability level acquiring means can take the reliability level from each of said plurality of base stations received by said receiving means as the reliability level with respect to each of said plurality of base stations. In particular, when the parameters are acquired in each of said plurality of base stations, transmission power control can be performed with higher precision in the mobile station.
Furthermore, the following characteristics can be appended to the above-described mobile station.
The mobile station further comprises weighting means for weighting each transmission power control signal received by said receiving means by the reliability level of a corresponding base station, and said object signal acquiring means can generate said object signal by combining the plurality of transmission control signals weighted by said weighting means.
The object signal acquiring means can select one from among said plurality of transmission power control signals to be said object signal.
The control content deciding means can compare said object signal determined by said object signal acquiring means with two preset threshold values, and select and decide from among the three stages of increase, maintenance or decrease of the transmission power as said control content based on the results of this comparison.
The control content deciding means can take, as the control content, control performed by changing the transmission power by a control amount in accordance with said object signal determined by said object signal acquiring means.
Additionally, in order to resolve the above-described problems, a base station of the present invention is a base station for performing the above-described transmission power control method wherein a parameter is acquired in said reliability level acquiring step for each of said plurality of base stations based on an upward signal from said mobile station; comprising parameter acquiring means for acquiring said parameter with respect to said mobile station based on an upward signal from said mobile station; and transmission means for transmitting said parameter acquired by said parameter acquiring means to said mobile station. By using the parameter acquired in this base station in the mobile station, transmission power control can be performed with higher precision in the mobile station.
Additionally, in order to resolve the above-described problems, a recording medium of the present invention contains a program to be run in a mobile station simultaneously connected to a plurality of base stations; the program comprising a receiving step for receiving a transmission power control signal form each of said plurality of base stations; a reliability level acquiring step for acquiring a reliability level with respect to each of said plurality of base stations; an object signal acquiring step for determining an object signal from said reliability levels acquired in said reliability level acquiring step and said plurality of transmission power control signals received in said receiving step; a control content deciding step for deciding a control content of the transmission power of said mobile station based on said object signal determined in said object signal acquiring step; and a control step for controlling the transmission power of said mobile station in accordance with said control content decided by said control content deciding step. By running the program recorded in this recording medium in the mobile station, the object signal is determined in the mobile station in consideration of not only the plurality of transmission power control signals, but also of the reliability level of each of the plurality of base stations, and the transmission power control is performed on the basis of this object signal. Therefore, the influence of a transmission power control signal for which an error has occurred during transfer can be made small. As a result, transmission power control can be performed with high precision, and the degradation of the communication quality and the degradation of the subscriber capacity can be prevented.